Apparatus for measuring the velocity of flow of an electrically conductive fluid

ABSTRACT

An electromagnetic velocity meter in which an electrical signal is generated in response to relative motion between the velocity meter and a conductive fluid in a magnetic field generated by means in the velocity meter, to provide a measure of the velocity of the relative motion. The velocity meter is particularly useful as a ship&#39;&#39;s log. The velocity meter is circular in cross-section and has at least a pair of electrodes for sensing the electrical signal, the electrodes being disposed on the flow meter in a manner to minimize error in the velocity readings of the meter. This is accomplished by providing an electrode configuration on the meter wherein the electrodes project outwardly from the probe surface.

[451 Aug. 12, 1975 1 APPARATUS FOR MEASURING THE VELOCITY OF FLOW OF AN ELECTRICALLY CONDUCTIVE FLUID [75] Inventor: James J. Dar y, Jr., Rockville, Md.

[73] Assignee: Marsh-McBirney, Inc., Rockville,

[22] Filed: Sept. 21, 1973 [21] Appl. No.: 399,733

FOREIGN PATENTS OR APPLICATIONS 1,941,325 6/1970 Germany 73/194 EM 946,394 8/1956 Germany 73/194 EM Primary ExaminerCharles A. Ruehl Attorney, Agent, or FirmLawrence E. Laubscher 57 ABSTRACT An electromagnetic velocity meter in which an electrical signal is generated in response to relative motion between the velocity meter and a conductive fluid in a magnetic field generated by means in the velocity meter, to provide a measure of the velocity of the relative motion. The velocity meter is particularly useful as a ships log. The velocity meter is circular in crosssection and has at least a pair of electrodes for sensing the electrical signal, the electrodes being disposed on the flow meter in a manner to minimize error in the velocity readings of the meter. This is accomplished by providing an electrode configuration on the meter wherein the electrodes project outwardly from the probe surface.

1 Claim, 5 Drawing Figures APPARATUS FOR MEASURING TI-IE VELOCITY OF FLOW OF AN ELECTRICALLY CONDUCTIVE FLUID BACKGROUND OF THE INVENTION l. Field of the Invention This invention relates to a velocity meter for providing an electrical signal depending on the speed of movement thereof relative to an electrically conductive fluid, and more specifically, an electromagnetic ships log.

2. Description of the Prior Art Velocity meters for providing an electrical signal dependent on the speed of movement thereof relative to an electrically conductive fluid, such as water, are well known.

Such a velocity meter usually includes a probe which is adapted to be immersed in the conductive fluid, such as water, when the meter is used as a ships log or when stationary, as a current velocity sensor. A pair of electrodes are embedded in the surface of the probe, whose axis of revolution is perpendicular to the direction of flow of fluid sensed by the probe. The probe carries means for generating a magnetic field so that motion of the probe through water, for example, which is, of course, a conductive fluid, in the presence of the magnetic field generated from the probe will cause a voltage to exist between the electrodes which is proportional to the velocity of the flow of the fluid. This voltage can be monitored to provide an indication of the speed of the ship through the water, or the velocity of water past a stationary probe. In order to obtain this reading, the electrodes are arranged on each of the two leading quadrants of the surface of the probe.

By placing a further pair of electrodes at 90 increments and measuring the potential difference generated between these electrodes, it is possible to obtain the speed of the ship in a fore-aft direction, as well as the drift velocity of the ship in a lateral direction. By comparing the two potential differences, the drift angle can be simply computed. The electrodes are equally spaced about the periphery of the probe to provide a plurality of electrical signals dependent on the vector components of the speed of relative movement of the meter and the electrically conductive fluid.

The voltage present between each electrode pair will be a function of the cosine and sine of the angle formed by (I) that component of flow perpendicular to the longitudinal axis of the probe and (2) a reference line perpendicular to the longitudinal axis of the probe and in line with one electrode pair. If the absolute value of the velocity vector is plotted as a function of this angle through 360, the absolute value of the vector should ideally lie on a point of a circle depending on the particular angle of flow observed. In practice, however, an 8 to 10 percent error from the ideal value has been found to take place.

It is not exactly known why there is such an error in the velocity reading. The flow of any fluid in the boundary layer changes from a laminar nature to a turbulent nature shortly after it passes the widest chord of the cylindrical probe. Theoretically, this turbulent layer does not proportionally produce the same voltage as is produced in the leading semicircle. Thus, if the flow direction issuch as to place an electrode in this turbulent SUMMARY OF THE INVENTION As stated, the aforementioned problem of lack of true sine and cosine response in sensing the velocity components at each electrode of the velocity meter is believed to be due to a transition of fluid velocity in the boundary layer of the fluid flow at the electrodes, wherein the boundary layer changes from laminar to turbulent. In order to overcome this problem and to minimize the error noted, it has been determined that if the electrode configuration is changed so that in lieu of embedding the electrodes in the probe surface, but by enabling them to project outwardly therefrom for a predetermined distance, the error is minimized to an extent wherein it disappears and there is true sine and cosine response of the various velocity vectors measured by the velocity meters.

The invention will be better understood from the following description and claims, and from the accompanying drawings, wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front view in elevation of a probe of an electromagnetic ships log which has been in common use heretofore;

FIG. 2 is a partial side view in elevation of the probe shown in FIG. 1;

FIG. 3 is a graphic representation illustrating the ideal and actual conditions of velocity measurement utilizing the probe illustrated in FIGS. 1 and 2;

FIG. 4 is a front view in elevation of the probe of an electromagnetic ships log illustrating the present invention; and

FIG. 5 is a side view in elevation of the probe shown in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings in detail, wherein like numerals indicate like elements throughout the several views, and with particular reference to FIGS. 1 and 2, initially, the probe of a ships log or velocity meter having a generally circular cross-section is indicated by the numeral 10. The probe 10 is carried by ship so that water is caused to flow around the probe at right angles to its longitudinal axis as shown by the arrow X in FIG. 1.

The probe is provided with a coil assembly 12 for generating a magnetic field having a component at right angles to the general direction of flow, when energized. The coil assembly 12 does not comprise any portion of the instant invention and is well known in the art. A typical coil assembly which can be used with the probe of the present invention is described in detail in US. Pat. No. 3,668,931. When the velocity meter is in operation, the coil assembly 12 is energized by a power supply inside the ship via connecting wires to generate the required magnetic field.

Two pairs of electrodes l4, l6, and 18, 20 are usually disposed at the ends of two perpendicular diameters of the probe 10, respectively. Motion of the probe or log through water, which is a conductive fluid, in the presence of the magnetic field generated by the coil assembly 12, will induce a potential difference between the electrode pairs 14, 16 and 18, 20, the potential difference being proportionalto the velocity of flow of the conductive fluid. Further means (not shown) are provided so that the voltages developed can be monitored on the shipto provide an indication of the speed of the ship through the water.

With the' arrangement of the electrodes as shown in FIGS. 1 and 2, the arrow X represents the direction of flow of the conductive water past the probe, and the two perpendicular components of the flow, in the directions A and B, are measured. Thus, if the probe is positioned so that the direction A is the fore-and-aft direction of the ship, then the potential difference developed between the electrodes 14, 16 will be indicative of the velocity made along the ships heading, and that developed between the electrodes 18, 20 in direction B is indicative of the drift velocity of the ship. By comparing these two potential differences, the drift angle can be simply computed.

As illustrated in FIG. 3, the voltages sensed by each of the electrodes will be a function of the cosine and sine of the angle wherein 0 is the angle that an imaginary line intersecting an electrode pair makes with the direction of water flow. In other words, the velocity vector will be broken into two components and the two voltages (representing those two components) present at each electrode pair will be sensed and will be a function of either the sine or cosine of the angle 6. As illustrated in FIG. 3, the velocity vector impinging at any position on the probe, when plotted against 0 through 360, should ideally be graphically represented by the circle 20. In practice, however, an 8 to 10 percent error has been noted and the actual value of the velocity vector is represented in FIG. 3 by the dotted line 22, and as graphically illustrated, the dotted line falls within the circle 20.

It is not exactly known why there is an error in the velocity readings as indicated. It is believed, theoretically, that the flow of the conductive fluid changes from a laminar to turbulent nature shortly after it passes the widest chord of the cylindrical probe. Theoretically, this turbulent layer does not proportionally produce the same voltage as is produced in the leading semicircle. Thus, if the flow direction is such as to place an electrode in this turbulent area, a smaller voltage is produced than would be expected from just a cosine effect.

In order to obtain a true velocity reading as indicated by the circle 20 in FIG. 3, it has been found that by changing the electrode configuration on the probe as shown in FIGS. 4 and 5, a true velocity reading for all angles can be obtained. It has been found that by using an electrode configuration as shown in FIGS. 4 and 5, the error in the velocity reading is completely eliminated and the velocity vector will trace a perfect circle (such as circle 20 in FIG. 3) when plotted against the angle 0.

FIGS. 4 and 5 illustrate a probe of a ships log which is similar to the probe 10in FIGS. 1 and 2 in that it is substantially circular in cross-section and includes acoil assembly 12' for generating a magnetic field perpendicular to the axis of the probe 10. The electrodes embedded on the surface of probe 10' are substantially different however.

As illustrated in FIG. 4, the electrodes are arranged in opposing pairs as in the prior art, but project outwardly from the probe surface for a length of approximately 0.05 inch for each inch of probe diameter.

The electrode pair 18' and 20 measures the drift velocity of the ship whereas the electrodes 14' and 16 will give a true measure of the foreand-aft velocity of the ship through a sine and cosine response which is circular in accordance with the graphic representation 20 in FIG. 3.

The phenomenon of true velocity measurement using the probe illustrated in FIGS. 4 and 5 is not completely understood, but it is believed that the placement of the sensing electrodes outside of the boundary layer of the conductive fluid along the probe surface minimizes the effect of decreased emf caused by the flow changing from laminar to turbulent adjacent to the probe in the two aft quadrants.

Although the invention has been described with reference to a probe for use as a ship s log, with the seawater as the conductive fluid, the invention may equally well be used to measure the velocity of flow of any electrically conductive fluid.

What is claimed is:

l. A velocity meter for measuring the relative velocity of an electrically conductive fluid, comprising a. a cylindrical probe adapted for insertion into the fluid normal to a given direction of relative movement (X) between said probe and said fluid;

b. means contained within said probe for producing in the fluid a magnetic field the axis of which is parallel with the longitudinal axis of said probe; and

c. signal generating means for providing an electric signal dependent upon the relative velocity of movement between said probe and the fluid, said signal generating means including two pairs of diametrically arranged electrodes mounted on 'the external circumferential surface of said probe, said electrodes lying in a plane normal to the axis of said probe, the electrodes of one electrode pair being disposed at the ends of a first diameter of said probe, respectively, the electrodes of the other electrode pair lying at the ends of a second probe diameter normal to said first diameter, respectively, each of said electrodes projecting radially outwardly from the external circumferential surface of said probe, each of said electrodes projecting outwardly from the circumferential surface of said probe a distance of approximately 0.05 inch per inch of probe diameter. 

1. A velocity meter for measuring the relative velocity of an electrically conductive fluid, comprising a. a cylindrical probe adapted for insertion into the fluid normal to a given direction of relative movement (X) between said probe and said fluid; b. means contained within said probe for producing in the fluid a magnetic field the axis of which is parallel with the longitudinal axis of said probe; and c. signal generating means for providing an electric signal dependent upon the relative velocity of movement between said probe and the fluid, said signal generating means including two pairs of diametrically arranged electrodes mounted on the external circumferential surface of said probe, said electrodes lying in a plane normal to the axis of said probe, the electrodes of one electrode pair being disposed at the ends of a first diameter of said probe, respectively, the electrodes of the other electrode pair lying at the ends of a second probe diameter normal to said first diameter, respectively, each of said electrodes projecting radially outwardly from the external circumferential surface of said probe, each of said electrodes projecting outwardly from the circumferential surface of said probe a distance of approximately 0.05 inch per inch of probe diameter. 